We present Spitzer 8 \mu m transit observations of the extrasolar planet HD 149026b .
At this wavelength , transit light curves are weakly affected by stellar limb-darkening , allowing for a simpler and more accurate determination of planetary parameters .
We measure a planet-star radius ratio of R _ { p } / R _ { \star } = 0.05158 \pm 0.00077 , and in combination with ground-based data and independent constraints on the stellar mass and radius , we derive an orbital inclination of i = 85 \mbox { $ . ^ { \circ } $ } 4 ~ { } ^ { +0 \mbox { $ . ^ { \circ } $ } 9 } _ { -0 \mbox { $ . ^ { % \circ } $ } 8 } and a planet radius of R _ { p } = 0.755 \pm 0.040 ~ { } R _ { J } .
These measurements further support models in which the planet is greatly enriched in heavy elements .